(1) Field of the Invention
The present invention relates to the field of anti-torque devices for rotary wing aircraft and it is intended more particularly for fitting to compound aircraft, i.e. aircraft having at least one rotary wing and at least one fixed wing.
The present invention relates to a compound aircraft having an additional anti-torque device. The additional anti-torque device provides additional torque that adds to the main torque provided by a main anti-torque device of the compound aircraft for the purpose of opposing rotor torque. The rotor torque is due to the reaction of the main rotor of the aircraft to the driving torque used for rotating the main rotor. Specifically, rotor torque tends to turn the aircraft fuselage with yaw movement opposite to the yaw movement of the main rotor. Consequently, a main anti-torque device is provided to subject the fuselage of the aircraft to yaw movement under the action of a main torque that is in the same direction as the driving torque.
(2) Description of Related Art
Rotary wing aircraft are aircraft that differ from other powered aircraft mainly in their ability to fly both at high speed in cruising flight and also at low speed, and in hovering flight. This ability is provided by fitting the aircraft with at least one rotary wing having an axis of rotation that is substantially vertical. The rotary wing is situated above a fuselage of the aircraft and is referred to as the “main rotor”. The main rotor provides the aircraft with at least part of its lift and with propulsion.
A rotary wing aircraft is generally characterized by three reference directions, a longitudinal direction X extending from the front of the aircraft towards the rear of the aircraft, an elevation direction Z extending upwards perpendicularly to the longitudinal direction X, and a transverse direction Y extending from left or right perpendicularly to the longitudinal and elevation directions X and Z.
The longitudinal X is the roll axis of the aircraft, the transverse direction Y is its pitching axis, and the elevation direction Z is its yaw axis. The axis of rotation of the main rotor is close to the yaw axis of the aircraft.
A main rotor has a plurality of blades and it is driven in rotation by a power plant of the aircraft via a main power transmission system. In order to be provided with balance about the yaw axis, the aircraft is provided with main anti-torque device creating a main torque about the yaw axis. The main torque serves firstly to oppose and counterbalance the rotor torque, and secondly to provide the aircraft with maneuverability about its yaw axis, in particular in hovering flight or during specific stages of flight.
There exist various configurations for main anti-torque devices for rotary wing aircraft.
By way of example, a main anti-torque device may be constituted by an auxiliary rotor generally situated at the rear of the aircraft, at the end of a tail boom of the aircraft. The auxiliary rotor may have an axis that is fixed or that can be pivoted, and it is driven in rotation by the power plant of the aircraft by means of an auxiliary power transmission system. A main anti-torque device may also be constituted by a jet of air oriented mainly in the transverse direction Y and generally situated at the end of the tail boom of the aircraft. In these examples, the main anti-torque device creates a transverse force on the tail boom of the aircraft and consequently generates a main torque about the yaw axis.
In another example, a main anti-torque device is constituted by two propellers situated transversely on either side of the fuselage of the aircraft. These two propellers are driven in rotation by the power plant of the aircraft via an auxiliary power transmission system and they create longitudinal forces. These two propellers thus provide the aircraft with some or all of its propulsion, depending on the stage of flight of the aircraft. A difference between the longitudinal forces created respectively by each of the propellers serves to generate a main torque about the yaw axis.
Whatever the main anti-torque device that is used, it is necessary to provide the anti-torque device with mechanical power in order to create the required main torque. This mechanical power driving the anti-torque device is in addition to the mechanical power needed for driving the main rotor. The power plant of the aircraft must thus deliver sufficient mechanical power to be able to drive simultaneously the main rotor and the main anti-torque device.
In addition, the power needed both by the main rotor and by the main anti-torque device varies depending on the stage of flight. Takeoff and hovering stages of flight are generally the stages of flight making the greatest demands in terms of power.
Furthermore, a rotary wing aircraft may have at least one fixed wing providing the aircraft with some or all of its lift when flying at high speed. A fixed wing may for example comprise two wing portions situated transversely on either side of the fuselage of the aircraft under the main rotor, each of which wing portions is referred to below as a “wing”. Such a rotary wing aircraft having two wings situated on respective sides of the fuselage of the aircraft is often referred to as a “compound aircraft”.
With this type of configuration, the wings of the aircraft are subjected to the aerodynamic influence of the main rotor. In particular in hovering flight or during takeoff, when each wing is completely immersed in the air stream generated in reaction to the lift of the main rotor, each wing is subjected to an aerodynamic drag force that is downwardly directed and thus gives rise to negative lift. This negative lift opposes in part the lift generated by the main rotor and therefore needs to be compensated by increasing the lift from the main rotor by an amount that is equal to and opposite from the negative lift. In order to increase the lift of the main rotor, the mechanical power delivered by the power plant of the aircraft must thus also be increased.
It can be seen that the already large requirement for mechanical power needed for hovering flight of a rotary wing aircraft, or indeed for takeoff, is made even larger for a compound aircraft. Furthermore, it is found that this mechanical power needed for hovering flight or takeoff is not limited to the power demand from the main rotor for lifting the weight of the aircraft, but is increased on two counts: firstly in order to drive the main anti-torque device; and secondly to compensate for the negative lift of each wing swept by the stream of air generated in reaction to the lift of the main rotor of a compound aircraft.
For simplification purposes, the term “air stream of the main rotor” is used below to specify the stream of air that is generated in reaction to the lift of the main rotor.
Consequently, the mechanical power that needs to be delivered in order to perform hovering and takeoff is often a parameter that determines the dimensions of the power plant of the aircraft and puts a limit on its general performance. Specifically, any reduction in this mechanical power needed for hovering flight or indeed for takeoff could be a significant source of improvement in the general performance for the aircraft. In particular, it is known that reducing the power needed for hovering by 1.5% for a given driving power limit enables the total takeoff weight to be increased substantially by 1%, and consequently enables the payload of the aircraft to be increased by about 2% to 3% for unchanged empty weight of the aircraft.
One way of reducing this need for power is to reduce the power requirement for performing the anti-torque function.
In order to provide the aircraft with lift, first means consist in using two main rotors turning in opposite directions so that their torques balance. There is then no need for the aircraft to have an anti-torque device. The two main rotors may be arranged transversely relative to the aircraft, or longitudinally, or indeed on the same axis.
However, using two rotors arranged transversely or longitudinally means that the two rotors need to be connected together by power transmission shafts, in order to synchronize their movements under all circumstances. That type of architecture is usually reserved for heavy aircraft, and is therefore not suitable for medium or light aircraft.
The use of two main rotors on the same axis, although applicable to any type of aircraft, is mechanically very complex, and in particular requires the use of two concentric rotary shafts and two systems for controlling the pitch of the blades of those main rotors. Furthermore, the blades of the two main rotors must never interfere with each other, whatever the vertical deformation or rotary movements to which they might be subjected, and that imposes additional criteria on the installation and/or the stiffness of the main rotors.
Second means consist in rotating a single main rotor, not by using a power transmission system, but by placing air thrusters on each of the blades. As a result, the main rotor rotates freely about its axis without any torque or else with torque that is very low and associated solely with friction between the shaft of the main rotor and its bearings. Nevertheless, a small amount of additional torque can be generated by the drive for essential accessories such as auxiliary electricity generators or hydraulic pumps, for example. In this way, a main anti-torque device that generates only a small amount of torque in yaw suffices to balance the residual torque from the main rotor and to provide the aircraft with yaw maneuverability under all flying conditions.
Nevertheless, high speed air propulsion takes place at low efficiency. Consequently, in spite of the need for a main anti-torque device that generates little torque, the power demanded of the power plant of the aircraft is greatly increased in order to compensate for the low efficiency. Furthermore, the very high level of noise generated by such high speed air propulsion constitutes a severe drawback for that architecture.
Third means consists in providing the tail boom of the aircraft with a shape that is asymmetrical and that generates a transverse force aerodynamically when it is swept by the stream of surrounding air, and in particular the stream from the main rotor. The transverse aerodynamic force serves to create yaw force that opposes the rotor torque, in part.
However, the asymmetrical shape is not sufficient to balance all of the rotor torque, but only serves to reduce the mechanical power needed by the main anti-torque device. A main anti-torque device is thus still necessary, in particular at low speeds. The asymmetrical shape can be replaced by blowing air from one of the sides of the tail boom.
For compound aircraft, it is also possible in known manner to reduce the negative lift of the wings. For this purpose, each wing of the aircraft is provided with movable flaps, e.g. at its trailing edge. The movable flaps then serve to adjust the lift of each movable assembly formed by the wing and the flap(s) it includes.
In the description below, the term “wing-and-flap assembly” is used to designate the assembly constituted by the wing and the flap(s) it includes on either side of the longitudinal direction X. One wing-and-flap assembly is thus situated on a first side of the fuselage of the compound aircraft relative to its longitudinal direction X, and another wing-and-flap assembly is situated on a second side of the fuselage relative to the longitudinal direction X.
While the aircraft is in cruising flight and flying at high speed, each movable flap is generally positioned in continuity with the profile of the wing at a deflection angle that is small or zero, thereby optimizing the aerodynamic lift of the wing and minimizing its interfering aerodynamic drag.
Furthermore, asymmetrical movement of the flaps on either side of the fuselage enables the aircraft to be piloted in roll. Specifically, the aerodynamic lift forces of each wing-and-flap assembly are then different on either side of the fuselage leading, amongst other things, to the aircraft being moved about its roll axis.
At low speeds, and in particular during hovering flight and while taking off, the flaps may be directed downwards, forming an angle close to ninety degrees (90°) relative to the wing, so as to take the rear portion of the wing-and-flap assembly as constituted by the flaps out of the air stream from the main rotor. The flaps may also be slidably mounted and retracted into the inside of the wing. The area of each wing-and-flap assembly that is exposed to the stream of air from the main rotor is thus reduced, thereby reducing the negative lift induced by the wing-and-flap assembly. Consequently, the power needed by the main rotor is reduced by the presence of the movable flaps, and likewise the power required from the power plant of the aircraft is reduced. The movements of the movable flaps may be controlled by the pilot of the aircraft or possibly by an autopilot of the aircraft.
Nevertheless, negative lift continues to be induced by the stream of air from the main rotor on each of the wing-and-flap assemblies. Specifically, the size of the flaps is limited by constraints concerning bulk, strength, and additional weight. Consequently, the power needed by the main rotor and the power needed by the main anti-torque device are still greater than for an equivalent aircraft that does not have a wing.
It is thus advantageous to propose means enabling the mechanical power needed by the main anti-torque device to be reduced so as to be able to reduce the mechanical power demanded of the power plant of the rotary wing aircraft.
By way of example, the following documents are known: U.S. Pat. No. 2,575,886; US 2008/0272244; U.S. Pat. No. 4,928,907; and GB 570 455; which all describe an aircraft having a lift rotor, two wings situated on either side of a fuselage, and possibly one or two propulsive propellers. None of those documents describes an aircraft having an anti-torque tail rotor. Specifically, the anti-torque devices in those aircraft make use of the stream of air from the lift rotor which sweeps over aerodynamic elements in order to generate aerodynamic forces and consequently torque opposing the rotor torque.
Thus, in Document U.S. Pat. No. 2,575,886, the aircraft has a multitude of adjustable flaps on each wing and on the fuselage. According to Documents US 2008/0272244, U.S. Pat. No. 4,928,907, and GB 570 455, the wings can be adjusted as a whole, and each wing has at least one flap that is adjustable relative to the wing. Adjusting the flaps and/or the wings then makes it possible while they are being swept by the air stream from the main rotor, to generate such aerodynamic forces and consequently to generate the torque for opposing the rotor torque. The flaps and/or wings may be identical on either side of the fuselage.
Furthermore, Documents US 2006/0157614, JP 2003/220999, and US 2005/0151001 form part of the technological background of the invention.